1. Field of the Invention
The present invention relates to a pressurizing device, an image forming apparatus, and a control method for the pressurizing device.
2. Description of the Related Art
Conventionally, image forming apparatuses using an intermediate transfer body, such as a tandem color image forming apparatus, have a problem that in the event of a change in the speed of the intermediate transfer body, a formed image has irregular color or lines, resulting in deterioration in image quality. The change in the speed of the intermediate transfer body occurs, for example, when a sheet runs into a nip between the intermediate transfer body and a roller. The force of impact on a sheet running into the nip is unsteady and transient and has broad frequency characteristics, and therefore is difficult to be suppressed by the speed control of the intermediate transfer body. As a conventional technology for coping with this, there is proposed a technology to control the pressure produced in the nip (the nip pressure).
For example, Japanese Unexamined Patent Application Publication No. 2010-151983 has disclosed a technology to keep the pressure low before a medium runs into the nip and then increase the pressure after the medium has run into the nip. However, in this technology, the pressure is applied to a fixing nip when the medium runs into the fixing nip; therefore, it is not possible to completely suppress the force of impact on a sheet running into the nip.
Furthermore, Japanese Unexamined Patent Application Publication No. 05-289569 has disclosed a technology to statically adjust the transfer pressure produced in a nip by moving the position of a secondary transfer roller according to the size or thickness of a sheet. However, in this technology, it is difficult to control the pressure according to the temperature characteristics or individual difference of the roller.
Moreover, Japanese Unexamined Patent Application Publication. No. 2014-038201 has disclosed a technology to weaken the contact pressure between a pair of registration rollers just before a sheet goes through the pair of registration rollers and suppress vibration produced when the sheet goes through the pair of registration rollers while ensuring the nip pressure required to convey the sheet.
In any of these conventional technologies, a controlled object is either the pressure (the nip pressure) or the roller position. In the case where the pressure is a controlled object, a time from when a sheet runs into a nip till when an image is transferred onto the sheet is generally about five to ten milliseconds; there is a problem that it is difficult to perform pressure control in such a short time. On the other hand, in the case where the roller position is a controlled object, rollers are an elastic body, and the elastic modulus varies with environmental changes and aged deterioration; therefore, mere is a problem that for example, even if the roller position is controlled on the basis of the amount of roller deformation, it is difficult to strictly control the nip pressure.